ISL62881_14 Datasheet, PDF (17/35 Pages) Intersil Corporation – Single-Phase PWM Regulator for IMVP-6.5 Mobile CPUs and GPUs

ISL62881, ISL62881B
TABLE 3. ISL62881 OCP THRESHOLD AND OVERSHOOT REDUCTION
FUNCTION
R_comp
OCP
MIN NOMINAL MAX THRESHOLD
(kΩ)
(kΩ)
(kΩ)
(µA)
none
none
20
305
400
410
22.67
205
235
240
20.67
155
165
170
18
104
120
130
20
78
85
90
22.67
62
66
68
20.67
45
50
55
18
OVERSHOOT
REDUCTION
FUNCTION
Disabled
Enabled
The default OCP threshold is the value when R_comp is not
populated. It is recommended to scale the droop current I_droop
such that the default OCP threshold gives approximately the
desired OCP level, then use R_comp to fine tune the OCP level if
necessary.
For overcurrent condition above 2.5x the OCP level, the PWM
output will immediately shut off and PGOOD will go low to
maximize protection. This protection is also referred to as
way-overcurrent protection or fast-overcurrent protection, for
short-circuit protections.
The ISL62881 will declare undervoltage (UV) fault and latch-off if
the output voltage is less than the VID set value by 300mV or
more for 1ms. It’ll turn off the PWM output and de-assert
PGOOD.
The ISL62881 has two levels of overvoltage protections. The first
level of overvoltage protection is referred to as PGOOD
overvoltage protection. If the output voltage exceeds the VID set
value by +200mV for 1ms, the ISL62881 will declare a fault and
de-assert PGOOD.
The ISL62881 takes the same actions for all of the above fault
protections: de-assertion of PGOOD and turn-off of the high-side
and low-side power MOSFETs. Any residual inductor current will
decay through the MOSFET body diodes. These fault conditions
can be reset by bringing VR_ON low or by bringing V_DD below the
POR threshold. When VR_ON and V_DD return to their high
operating levels, a soft-start will occur.
The second level of overvoltage protection is different. If the
output voltage exceeds 1.55V, the ISL62881 will immediately
declare an OV fault, de-assert PGOOD, and turn on the low-side
power MOSFETs. The low-side power MOSFETs remain on until
the output voltage is pulled down below 0.85V when all power
MOSFETs are turned off. If the output voltage rises above 1.55V
again, the protection process is repeated. This behavior provides
the maximum amount of protection against shorted high-side
power MOSFETs while preventing output ringing below ground.
Resetting VR_ON cannot clear the 1.55V OVP. Only resetting V_DD
will clear it. The 1.55V OVP is active all the time when the
controller is enabled, even if one of the other faults have been
declared. This ensures that the processor is protected against
high-side power MOSFET leakage while the MOSFETs are
commanded off.
Table 4 summarizes the fault protections.
TABLE 4. FAULT PROTECTION SUMMARY
FAULT TYPE
FAULT DURATION
BEFORE
PROTECTION
PROTECTION
ACTION
FAULT
RESET
Overcurrent
Way-Overcurrent
(2.5xOC)
120µs
<2µs
PWM tri-state, VR_ON
PGOOD
toggle or
latched low VDD toggle
Overvoltage +200mV 1ms
Undervoltage -300mV
Overvoltage 1.55V
Immediately
Low-side
VDD toggle
MOSFET on
until V_core
<0.85V, then
PWM tri-state,
PGOOD
latched low.
Current Monitor
The ISL62881 provides the current monitor function. The IMON
pin outputs a high-speed analog current source that is 3 times of
the droop current flowing out of the FB pin. Thus as shown by
Equation 6.
I_IMON = 3 × I_droop
(EQ. 6)
As Figures 1 and 2 show, a resistor R_imon is connected to the
IMON pin to convert the IMON pin current to voltage. A capacitor
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information.
voltage
The
information be referenced to VSS_SENSE.
The IMON pin voltage range is 0V to 1.1V. A clamp circuit
prevents the IMON pin voltage from going above 1.1V.
Adaptive Body Diode Conduction Time
Reduction
In DCM, the controller turns off the low-side MOSFET when the
inductor current approaches zero. During on-time of the low-side
MOSFET, phase voltage is negative and the amount is the
MOSFET R_DS(ON) voltage drop, which is proportional to the
inductor current. A phase comparator inside the controller
monitors the phase voltage during on-time of the low-side
MOSFET and compares it with a threshold to determine the
zero-crossing point of the inductor current. If the inductor current
has not reached zero when the low-side MOSFET turns off, it’ll
flow through the low-side MOSFET body diode, causing the phase
node to have a larger voltage drop until it decays to zero. If the
inductor current has crossed zero and reversed the direction
when the low-side MOSFET turns off, it’ll flow through the
high-side MOSFET body diode, causing the phase node to have a
spike until it decays to zero. The controller continues monitoring
the phase voltage after turning off the low-side MOSFET and adjusts
the phase comparator threshold voltage accordingly in iterative steps
such that the low-side MOSFET body diode conducts for
approximately 40ns to minimize the body diode-related loss.
17
FN6924.3
June 16, 2011